"We take this day to remember that all of us are equal, regardless of race, gender, sexual orientation, gender identity, age, religion, or magical ability.

We write “mudblood” on our arms this day not only to remember the pain inflicted on Hermione Granger by Bellatrix Lestrange for being born to Muggles, but to acknowledge the hardships people have faced for being perceived as different.We invite Muggles and Wizards alike to join us in writing “mudblood” on our arms as a symbol to show that while we are all different, we are all equal."

Yes, I know that I was supposed to write it in my arms, but I forgot and wrote in my knuckles, but I liked this picture anyway, and I think the message was passed the same way

Being 'personally offended' is almost always unjustified. If something is wrong or untrue, it's simply wrong or untrue. And because morality is merely circumstantial, and truth is scientific and therefore liable to be disproved, emotionally attaching oneself to an absolute sense of right and wrong or belief in an apparent fact will inevitably lead to 'personal offense', and become a hindrance to social justice and intellectual development.

The answer to both your questions is politics. You can offend them, but society will decry you because they have succeeded in hammering their 'values' in to the public psyche.

Human rights is a made-up concept, but an essential one for civilization to exist. There is no more inherent value in my existence as there is in some rock on Mars. We are all made of star stuff. But because we are here, and have somehow agreed between us that we should appreciate existence (not just our existence, but the entire cosmos) we come together and learn how to live with each other given our psychosociological commonalities. That is where we derive 'rights' from. Any agreed-upon ground rule that is good for our collective long-term survival and development as a species, nay, a biosphere. But one must remember that these rights, and morality broadly-speaking, have, are, and will always be evolving to suit our long-term goal of flourishing as an enlightened civilization. It is important to realize that morality is made-up, because only then can we bring ourselves to change as we see fit.

Religions generally claim a 'sacred' set of immutable moralities that may have sounded like a good idea at the times they were invented either because we didn't know any better and/or because some prick claimed divine providence of them, and that is why progress clashes with them.